Send changedAttributes to Backbone.sync

[gblache]

• When changedAttributes is called from within Backbone.sync
  it returns false as the change method overwrites
  _previousAttributes. This makes it hard to send only
  the updated attributes to the server when a model is
  updated.

[eoin]

I'd love to see this merged.

[XiXora]

Sending only updated attributes? Sounds like PATCH method to me. :)
http://tools.ietf.org/html/rfc5789

[jashkenas]

This has been discussed several times before -- check the issues. Backbone by default PUTs the full representation of the model -- if you'd like to only send the change set, feel free to override sync or save.

[eoin]

Understood, however overriding sync to only send changed attributes is currently quite tricky. This change would make it much more straightforward.

[matjaz]

This is indeed issue. Calling changedAttributes() inside CUSTOM sync ALWAYS returns false.

[walling]

At least an example how to go about doing this in a custom sync would be preferable.

[mattdiamond]

This could work as a makeshift solution:

function sync (method, model, options){
  var newData = {};
  for (var prop in options.changes){
    if (options.changes.hasOwnProperty(prop)){
      newData[prop] = model.get(prop);
    }
  }
  /* save newData to the server */
}

[paulcollinsiii]

Another way of kinda doing the same thing:

/**
 * This assumes we're just overriding the sync method of a single model
 * and that we're only concerned with poking at changed data for update
 * events sent to the server
 */
sync: function (method model options){
  if(method === "update"){
     var changedData = {};
     _.each(options.changes, function(changed, key){
          if(changed){ changedData[key] = model.get(key); }
     });
     // If you set data, you need to declare your content type
     // Note I use PATCH because my backend uses tastypie
     // it's not strictly required though
     options = _.extend(options, {type: "PATCH",
                                  contentType: 'application/json',
                                  data: JSON.stringify(changedData)});
   }
   Backbone.sync(method, model, options);
} 

The "sneaky" bit here is putting data in the options. Keep in mind that if you have to do emulateJSON that changes what you'll have to set your options.data to.